The invention relates to a method for applying a coating to a substrate by means of plasma spraying with the use of a laser beam, which is suitable for coating a very wide variety of substrates, in particular for improving the properties, such as for example hardness, wear resistance and corrosion resistance, and which can be achieved with high coating capacities.
U.S. Pat. No. 3,310,423 has disclosed a flame-spraying method in which a pulsed laser beam is to be used for assistance in order to improve or increase the adhesion of the coating and its density.
In that document, it is proposed to direct the beam of a laser, which is in temporally short pulses, into a spray jet, specifically in such a way that the particles are reheated before they strike the surface to be coated. The document furthermore proposes to increase the particle temperature to such an extent that the surface is softened by heat transfer from the particles and forms an alloy with the particle material. In the case of a multi-layer build-up, the intention is for the surface of the top layer to be partially melted and fused to the particles sprayed on.
However, with the performance parameters indicated, only a limited amount of energy can be made available to increase the temperature in pulsed operation of the laser, so that only relatively low coating rates can be achieved or the desired level of success cannot be achieved.
Moreover, it is impossible to influence other properties of the coating applied which are again desirable in different respects for a very wide variety of coating materials. For example, it is possible that during cooling phases, microstructural or crystalline structures which do not have the desired properties, or have them only to an insufficient extent, will be formed. When applying a coating which contains at least Al.sub.2 O.sub.3, the .gamma. phase may be formed, the properties of which are far removed from those of the preferred .alpha. phase. Similar effects also occur for metals or such alloys. Owing to its high melting point, Al.sub.2 O.sub.3, for example, cannot be processed at all to give usable layers by flame spraying, even with the laser assistance proposed in that document.